Cross-fade is a general term for the situation where moving images in film or video sequences dissolve gradually between one scene and another. Cross-fades are a common video editing effect and typically are used to create a softer transition between scenes than a cut.
FIG. 1 illustrates an exemplary cross-fade over 5 images between a first image sequence, or scene, A and a second image sequence, or scene, B. Typically a cross-fade will occur over more than five images (four periods) in a sequence of images, and so FIG. 1 is intended to be merely illustrative of the principles involved in a cross-fade. In the first image of the cross-fade, image 1, the image from sequence A is contributing 100% of the picture information and the image from sequence B is contributing 0% of the picture information. As the cross-fade progresses, the proportion of the picture information contributed by the corresponding image from sequence B increases while the proportion of the picture information contributed by the corresponding image from sequence A decreases. Thus the middle image of the cross-fade, image 3, has an equal contribution from the corresponding images in sequences A and B. In the last image of the cross-fade, image 5, the image from sequence A is contributing 0% of the picture information and the image from sequence B is contributing 100% of the picture information. Clearly, there may be motion in either or both of the first or the second image sequence during a cross-fade, but the principle of variable contributions from each of the first and second image sequences remains.
A special case of cross-fades occurs when either the original or the target scene is a solid colour, typically black. These special cases are sometimes called fade-in and fade-out.
Increasingly it is desirable to analyse image sequences in order to detect cross-fades.
One method previously proposed in “Feature-based algorithms for Detecting and Classifying Scene Breaks” Ramin Zabih, Justin Miller and Kevin Mai. ACM Journal of multimedia systems 7(2) pp 119-128 March 1999, relies on searching a sequence of images for appearing and disappearing edges or lines, in order to detect cross-fades.
“Analysis-by-synthesis Dissolve Detection” Michele Covell, S. Ahmad. IEEE International Conference on Image Processing, Rochester N.Y. September 2002 discloses an alternative technique in which a “synthesis” dissolve is carried out between a previous image and the current image in a sequence and the result compared with intervening images in the sequence to determine whether a cross-fade has taken place.
A combination of these ideas is presented in “Reliable Dissolve Detection” Rainer Leinhart, Storage and retrieval for Media Databases 2001, Proc. SPIE 4315, pp 219-230, January 2001.
These prior art techniques involve considerable processing with multiple passes through the video data and are only suitable for off-line applications. In addition, the detection rates are generally poor.